1. Field of the Invention
The present invention relates to an exhaust system that is mounted on a motor vehicle such as, for example, a motorcycle for discharging exhaust gases emitted from a combustion engine of the motor vehicle.
2. Description of the Prior Art
The conventional exhaust system is so designed and so structured that a catalyst is built in a muffler and exhaust gases emitted from the combustion engine are, after having been substantially purified with the use of a catalytic converter disposed inside the muffler, discharged to the atmosphere. In view of upwelling of consideration to the environment in these years, the exhaust system disclosed in, for example, the Japanese Patent No. 3026684, published in Jan. 28, 2000, includes the catalytic converter positioned in an intermediate portion of an exhaust passageway, and an exhaust gas sensor positioned upstream of the catalytic converter with respect to the direction of flow of the exhaust gases, so that with the catalyst held in the vicinity of the exhaust ports, the catalyst can readily be activated immediately after the cold start.
It has, however, been found that since in the exhaust system of the type described above, a plurality of exhaust passages are merged at a location upstream of the catalytic converter to provide a single exhaust passage, a portion of the exhaust system, where the plural exhaust passages reside, tends to be shortened and, therefore, an increase of the engine output at a medium speed region is not sufficient. In view of this, the Japanese Laid-open Patent Publication No. 2006-077727, published in Mar. 23, 2006, suggests dividing a portion of the exhaust passage upstream of the catalytic converter into two zones by means of a partition plate so that the engine output at the medium speed region can be increased. However, if the exhaust passage is divided by the partition plate such as suggested in the Japanese Laid-open Patent Publication No. 2006-077727 mentioned above, an exhaust gas sensor need be installed in each of passages so divided by the partition plate, resulting in increase of the number of component parts, weight and cost.